The present invention relates generally to optical fiber amplifiers and, more particularly, to techniques for coupling pump energy into the pump core layer of an optical fiber amplifier.
Fiber amplifiers are commonly used in many applications, including telecommunications applications and high power military and industrial fiber optic applications. For example, both U.S. Pat. No. 5,946,130, issued Aug. 31, 1999 to Rice and U.S. Pat. No. 5,694,408 issued Dec. 2, 1997 to Bott et al. describe many such applications in which fiber amplifiers are employed including the processing of materials, laser weapon and laser ranging systems, and a variety of medical and other applications.
Optical fiber amplifiers are designed to increase the power output levels of the signals propagating therealong. One conventional optical fiber amplifier design is an end pumped dual clad fiber. Referring to FIG. 1, the dual clad fiber 10 has a single mode signal core 12, a multi-mode pump core 14 surrounding the signal core, and an outer cladding layer 16 surrounding the pump core for confining pump energy within the pump core such that signals propagating through the signal core are amplified. The signal core will typically be doped with one or more rare earth elements such as, for example, ytterbium, neodymium, praseodymium, erbium, holmium or thulium. In operation pump energy is coupled into the pump core at the input end 18 of the fiber. The pump energy then propagates along through the pump core until it is absorbed by the dopant in the signal core, thus amplifying signals propagating through the signal core. Although dual clad fibers can have different sizes, one typical dual clad fiber includes a signal core that has a diameter of 8-10 xcexcm and a pump core that has cross-sectional dimensions of 100-300 xcexcm. End pumped dual clad fiber amplifiers of this size can typically reach fiber energy power levels of 115 W.
To allow the largest amount of pump energy as possible to be coupled into the end of the fiber, the size of the pump core is generally made as large as practical. The size of the pump core, however, is limited by the requirement to maintain a significant absorption of pump energy per unit length of fiber. While a design that introduces pump energy into the end of the fiber has led to great increases in output power levels, the practical limits have essentially been reached for pump arrays of typical power output. Facing this problem, a number of alternative pumping techniques have been developed. For example, U.S. Pat. No. 5,854,865 issued Dec. 29, 1998 to Goldberg discloses a fiber amplifier having a v-shaped notch cut into the pump core through the cladding layer. Pump energy can then be reflected or refracted from one of the angled faces of the v-shaped notch so as to be injected directly into the pump core. Another technique involves the use of a fiber amplifier having portions of the cladding and the pump core removed. The fiber amplifier is then spooled between two reflective elements and pump energy introduced into the region between the reflective elements. The pump energy is then repeatedly reflected by the reflective elements in order to amplify signals propagating through the signal core.
The current techniques, while achieving some level of success, also have their drawbacks. They can require extensive and tightly controlled processing. Also, they are generally not easily amenable to volume manufacturing and scaling. Thus, it would be advantageous to provide an inexpensive optical fiber amplifier with a relatively straightforward design that is capable of being fabricated in mass quantities while addressing each of the other aforementioned features.
An optical fiber amplifier is therefore provided that employs a prism to optimally couple pump energy into the pump core of a dual clad fiber. The optical fiber amplifier of the present invention consists of a dual clad fiber having an inner (e.g., signal) core, an outer (e.g., pump) core surrounding the inner core, and a cladding layer at least partially surrounding the outer core. Pump energy can then be injected into the outer core through the prism located adjacent to the outer core in order to amplify signals propagating through the inner core.
In one embodiment, the outer core, cladding layer, and the prism of the fiber amplifier are selected such that:
xcex1xe2x80x2 greater than xcex8c
where xcex1xe2x80x2 is the angle of refraction of the pump energy at the prism-outer core interface and xcex8c is the critical angle of incidence in the outer core with the cladding layer. The critical angle, xcex8c, is given by Snell""s Law as:
xcex8c=sinxe2x88x921[Ncl/Np]
where Ncl is a refractive index of the cladding layer and Np is a refractive index of the outer core. Calculating the acceptable angles of refraction, xcex1xe2x80x2, from the critical angle, xcex8c, the angle defined between a face of the prism through which the pump energy is injected and the surface of the outer core, xcex1, is given by Snell""s Law as:
xcex1=sinxe2x88x921[(Np/Npr)*sin xcex1xe2x80x2]
where Npr is a refractive index of the prism.
The prism of the fiber amplifier includes a first face proximate the outer core, a second face extending at an acute angle xcex1 from one end of the first face through which pump energy is injected and a third face extending outwardly at an acute angle xcex2 from another end of the first face. In one embodiment, the prism is selected such that angle xcex1 equals angle xcex2 such that pump energy can be introduced into the outer core through both the second and third faces so as to propagate in opposite directions through the outer core.
In another embodiment, a reflective surface is located adjacent to the third face of the prism. The reflective surface reduces loss by redirecting pump energy that would otherwise escape through the prism back into the outer core. In one embodiment, the second face of the prism includes an integral lens, such as an integral cylinder lens. The integral lens can collimate and/or focus the pump energy into the prism. In another embodiment, the prism extends circumferentially about the outer core. In a further embodiment, the refractive index of the prism is greater than the refractive index of the cladding layer.
In yet another embodiment, the optical fiber amplifier consists of the dual clad fiber disposed, typically in a spooled arrangement, on a support, such as a heat sink. A prism extends across the fiber at two or more locations along the length of the fiber such that pump energy can be injected into the outer core at each of these locations in order to amplify signals propagating through the inner core. In the embodiment in which the dual clad fiber is spooled on the support, the prism can extend thereacross such that the prism is disposed proximate opposed portions of the dual clad fiber. The support can define a groove through which the fiber extends. The prism can then extend across the groove so as to overlie the fiber while resting on a portion of the support in the vicinity of the groove.
The optical fiber amplifier of the present invention therefore offers a relatively straightforward design that permits pump energy to be readily injected into the outer core and, in turn, absorbed into the inner core. Additionally, the optical fiber amplifier is capable of being fabricated in mass quantities utilizing conventional manufacturing processes, thus avoiding extensive and tightly controlled processing.